FIG. 5 is a cross sectional view of a conventional thermal fuse. A couple of lead conductors having surface plating layers 2a formed thereon are connected to respective ends of fusible alloy 1 including tin through melting fusible alloy 1 by electrical welding or laser welding. Plating layer 2a is composed of tin or solder which includes 60 to 65 wt. % of tin and 40 to 35 wt. % of lead. Fusible alloy 1 is coated with flux 3 and is placed in tubular case 4 having openings at respective ends. The openings of case 4 are sealed with hard resin 5.
In the conventional thermal fuse constituted as above, when lead conductor 2 is connected to fusible alloy 1, not only fusible alloy 1 melts, but also material of plating layer 2a having a low melting temperature melt, such as tin and solder, melts. The tin and lead composing plating layer 2a diffuse into a connection portion between lead conductor 2 and fusible alloy 1, and slightly changes a melting temperature of the connection portion, thus causing a fusing temperature of the thermal fuse to vary.
Variation in the fusing temperature will be explained below.
Fusible alloy 1 including tin is composed of eutectic alloy including 63 wt. % of tin and 37 wt. % of lead and having a melting temperature of 183° C. Fusible alloy 1 may have its composition changed and include an appropriate amount of indium appropriately, thus allowing the melting temperature to range from 120° C. to 140° C. Fusible alloy 1 including tin and lead may include an appropriate amount of bismuth, thus allowing the melting point of the alloy 1 to range 95° C. to 165° C. As above, the melting temperature of fusible alloy 1 increases if the alloy includes a large proportion of tin and lead, but the melting point decreases if the alloy includes indium and bismuth.
When lead conductors 2 are connected to fusible alloy 1 including tin, tin and lead, materials of plating layer 2a, may diffuse into both ends of fusible alloy 1, thus changing the composition at the ends of the alloy to vary and increasing the melting temperature at the ends accordingly.